1. Field of the Invention
The present invention relates to a control circuit for battery charging and discharging. More particularly, the invention relates to a circuit for controlling the charging or discharging of a battery that is built into electronic equipment, and of an expansion battery that is provided separately from the built-in battery.
2. Description of the Related Art
Battery-powered electronic devices, such as portable telephones and portable computers, are extremely common and widespread. These electronic devices are often configured so as to allow an expansion battery to be attached in addition to the device's internal battery to extend operating time.
An example of a charging and discharging circuit for an internal battery is shown in FIG. 3. In the circuit shown in FIG. 3, an internal battery 10 and an expansion battery 12 are basically handled in parallel. In other words, charging and discharging of the internal battery 10 and the expansion battery 12 are performed at the same time.
Charging and discharging of the internal battery 10 and expansion battery 12 are controlled by a control circuit 14. The control circuit 14 monitors terminal voltages of the internal battery 10 and the expansion battery 12 with voltage sensors 16a, 16b. Amounts of charge in the internal battery 10 and in the expansion battery 12 are determined on the basis of these terminal voltages.
If it is judged that the internal battery 10 or the expansion battery 12 is fully charged, the control circuit 14 places an FET 18a (refer to FIG. 3) into a conductive state. In order to place the FET 18a into the conductive state, the control circuit 14 impresses a predetermined control signal onto a gate of the FET 18a. When the FET 18a enters the conductive state, discharging occurs from the internal battery 10 and expansion battery 12 through diodes 20a and 20b to an electric circuit 22. Power is supplied electric circuit 22, which may be a power drawing circuit of a portable telephone or a circuit such as a CPU in a notebook computer.
When discharging occurs in this manner, the internal battery 10 and the expansion battery 12 are connected in parallel so that discharging occurs simultaneously from both batteries.
On the other hand, when charging the batteries, the control circuit 14 places the FET 18a into a non-conductive state and an FET 18b into a conductive state. To place the FET 18b into the conductive state, the control circuit 14 impresses a predetermined control signal onto a gate of the FET 18b. When the FET 18b enters the conductive state, power is supplied from a power supply circuit 24 is to the internal battery 10 and the expansion battery 12 through respective diodes 20c and 20d. Thus, the internal battery 10 and the expansion battery 12 are also connected basically in parallel when being charged, and both batteries are therefore charged simultaneously.
Thus, according to the technique for charging and discharging batteries in the prior art, the internal battery 10 and the expansion battery 12 are parallely connected by diodes. Current therefore flows during both charging and discharging so that the remaining charge in both batteries is approximately equal.
More specifically, when battery discharging occurs, since the battery terminal voltage increases for the battery having a greater amount of charge, current flows from the battery having the higher terminal voltage because diodes 20a and 20b allow current to flow only from the battery having the higher terminal voltage.
On the other hand, when the batteries are charging, the internal resistance of the battery having the smaller amount of remaining charge lowers during charging. Therefore, after the internal battery 10 and expansion battery 12 are connected in parallel by diodes 20c and 20d, more charging current consequently flows to the battery having the lower internal resistance, in other words, the battery having the smaller amount of remaining charge. As a result, charging proceeds so that the amounts of charge are equal for internal battery 10 and expansion battery 12. In the invention of Japanese Patent Laid-Open Publication No. Hei 6-217464 charging is performed for a plurality of batteries from a battery closer to an internal battery so as to simplify battery removal, whereas discharging is performed from an external battery far from a computer.
In this manner, the charging and discharging operations in electronic equipment equipped with the internal battery 10 and the expansion battery 12 are performed simultaneously (in parallel) for the internal battery 10 and the expansion battery 12. Therefore, the amounts of charge in both batteries drop simultaneously with the use of the electronic equipment.
However, from the viewpoint of operating an electronic device, it is preferable that some charge remain in internal battery 10 when the amount of charge in the expansion battery 12 has reached 0, and further preferable to operate the electronic device using that power and to remove and charge the expansion battery 12 while using that power. This manner of operation would enhance the operability of such electronic equipment. In comparison, according to the art shown in FIG. 3, a current problem is that the use of the electronic equipment must often be temporarily halted when both the internal battery 10 and the expansion battery 12 simultaneously drop to an amount of charge of 0.
By maintaining an amount of charge in the internal battery even when the amount of charge in the expansion battery becomes empty, a system may be operated continuously without being halted.